Method of electroplating copper



Sept. 8, 1953 A. E. CHESTER METHOD oF ELECTROPLATING COPPER Filed July 17, 195o m .M R m J M n m 3 a Seton/a? Patented Sept. 8, 1 953 METHOD OF ELECTROPLATING COPPER Allan E. Chester, Highland Park, Ill., assignor to Poor & Company, Chicago, Ill., a corporation of Delaware Application July 17, 1950, Serial No. 174,243

This invention relates to a new and improved method of electroplating, and more particularly to a methodv of electrodepositing metals from a plating bath in a new and improved manner adapted to produce substantially uniform distribution on articles or objects of configurations such that theV ordinary type of electroplating causes uneven distribution of the electrodeposited metal.

It is well known that flat objects, when electroplated in the usual way in an electroplating bath, tend to build up a higher concentration of metal near the edges than in the middle. This is true to some extent in objects of other configurations and various devices have been proposed in order to overcome this difficulty.

One of the objects of the present invention is to provide a new and improved method for overcoming the difculty mentioned above and for electrodepositing metals from plating baths in a uniform manner.

Another object of the invention is to provide a new and improved method for electrodepositing metals on ilat sheets or other flat objects in a manner such that substantial uniformity of distribution is obtained.Y

v A still further object is the provision of a new and improved method for controlling thel distribution of a metal from a plating bath.

Other features and advantages of the invention will appear hereinafter by reference to the following description thereof, in conjunction with the accompanying drawings in which Fig. 1 illustrates diagrammatically one form of apparatus suitable for the practice of the invention;

Fig. 2 illustrates graphically one of the limits of the wave form of the plating current; and

Fig. 3 illustrates another limit of the wave form of the plating current in accordance with the preferred practice of the invention.

Generally stated, the invention involves a new and improved method of controlling the distribution of copper deposited on an electrically conducting surface from a plating bath by superimposing alternating current on' direct current within certain predetermined limits. The invention is especially applicable to electroplating operations in which the work or article is so shaped that the electrodeposited metal builds up in some areas more than others. The plating current for the purpose of this invention may be described as an asymmetric alternating current in which the negative peak value of the wave form varies from zero (as illustrated in Fig. 2)

2 Claims. (Cl. 204-52) 2 to -2/3 of the positive peak value (as illustrated in Fig. 3)

Referring to Fig. 1, the apparatus illustrated comprises a `suitable receptacle l containing the plating bath, provided with one or more electrodes, generally indicated vat 2, and a work piece or conducting surface to be plated, generally indicated at 3. As will be understood, the electrode 2 forms the anode and the work piece 3 the cathode.

The anode 2 is connected by suitable conductors to the secondary 4 of a power transformer having a core 5 and a primary 6. The primary 6 is connected to a regulating auto-transformer, generally illustrated at l, which in turn is connected to a suitable source of electrical energy, such as a 220 or 440 volt alternating current. Direct current is supplied by means of a low Voltage current generator, generally shown at 8, one side of which passes to the work piece 3 through a rheostat 9, and the other side of which is connected to the secondary 4 in the manner shown.

In Figs. 2 and 3 the wave form of the current is graphically illustrated in a conventional manner, the horizontal axis (X-X) representing time and the vertical axis amplitude. The peak values are calculated from the equation where E is voltage. For the wave form to be asymmetric, the negative peak value must drop below the X-axis. Thus, the wave form of Fig. 2 ybecomes asymmetric when the negative peak (Y1) falls below the X-axis and is a different value from the positive peak value (Y in Fig. 2, Z in Fig. 3). Fig. 3 represents the maximum negative value (Z1) which may be employed for the purpose of this invention. Any Wave form similar to Fig. 2 in which the negative peak value is positive or above the X-axis may be described as pulsating direct current.

In a broader sense, the apparatus described and the hook-up of the component parts is believed to represent a new and improved system for the electrodepostion of metals by the superimposition of an alternating current on a direct current, irrespective of whether the ratios of a1- ternating current to direct current are within the limits preferred for uniform distribution. Thus, this type of apparatus may be employed in plating articles of any conguration with other ratios of alternating current to direct current where the distribution of the electrodeposited metal is not an important factor. This type of apparatus is suitable, therefore, for electroplating operations by the superimposition of alternating current on direct current in such a manner as to form a, pulsating direct current.

The method of controlling distribution of the electrodeposited metal as described herein is especially suited for the formation of very thin. substantially uniform films or coatings of ametal on an electrically conductive surface. For example, it has,Y been found that this; invention is especially adapted for-forming coatings of copperon steel or cast iron objects in which the thick? ness of the coating is within the range of .000005Y inch to .00005 inch. Steel or-cast-iron.-obiectsly when plated with copper in accordance with the invention to the thickness described', are cially suitable for the additionof' coatings et vitreous enamels, as described and claimed in my copending application Seriat NIL 555,953, filed September 27, 1944 now Patent No. 2,615,- 836. The thickness and uniformity of the piate is an important factor because it has been found that if the electrodeposited plate is. insuiiicient,v the desired bonding action will not occur. The same is true if the thickness of the coatingis-:too great. Thus, this invention makes possible new and improved results in the art of enameling, as well as new and improved results` in control-` ling the distribution of metals by eiectrodeposi= tion.

The invention will be further illustrated. but. is, not limited by the following example:

Example I A typical copper plating bath which maybecmployed has the following composition:

Ounces per gallon Cu SO4.5H2O' 24` HaSO'i G Glue (optional) /A It will be understood that the invention is sus-- ceptilole to some variation and modification in the manner of its practical application.

The ratios of alternating to direct currentwhich are. preferably employed; in@ accordance with the invention in order toobtain the desired wave form, are about 2-4 volts (root meansquare) A. C.: 2.5 volts D. C., preferably at current densities from 2 to l5 arnp'ere's persquare foot. Alternating currents of different fre-,- quencies may be used, but it` isl preferablev to. employ either a 25 cycle or a 60 cyclel current. The plating time will vary depending., upon the. amount of metal which it is desired todeposit. Thus, for thicknesses of the type, previously mentioned for vitreous enamel work, a plating time from 8 seconds to 11/2 minutes is sufficient., Itwill be understood that higher current densities may be-used, and that the alternatingiandidiret'. current voltages may be larger-or sin-aller, provided that the ratio produces a predetermined wave form of the type previously; described. Thus, if the direct current is 0.5 volt, thcalternating current wouldl be correspondingly-reduced. in accordance with the foregoing ratio. If the direct current is 10 volts, the alternating current is increased in the foregoing ratio.

The plating current will be governed by; the surface area of the article being plated, the-size: of the bath, power facilities and other factors: which will be recognized by those skilledl in the art.

The expression copper plating bath i's-in'-,

tended to include plating baths of the type de-f scribed in Example I where other metals or salts thereof are present in minor proportion as Well as those in which such metals or salts are absent.

The invention provides a new and improved method for controlling the deposition of copper on electrically conducting surfaces. It is especially useful in' applying even, uniform coatings to articles having a con-figuration in which, by ordinary direct current plating methods, the deposited metal tends to build up in some areas more thanothers. Thus, it is especially important for electroplating flat surfaces when a thin plate of substantially uniform thickness is desiredi.

This application is a continuation-in-part of vcopending-application Serial No. 555,954,

filed September 27, 1944, now U. S. Patent No. 2go-15,192.

The invention is hereby claimed as follows:

LIn; a method of controlling the electrodepsition4 of copper from an acid copper sulfate plating: bath onto an electrically conducting objectrofa type-in which the copper would normally be deposited ingreater concentrations in some lareas than in; others by a direct plating current;v the step whichV comprises electrodepositing the.

copper-by-subjecting said object, in an acid-plating bath to a plating current having an asymmetric alternating wave-form comprising an alternating current superimposed on a direct current in the ratio of 244 volts R. M. S. to 2-.5 volts D. C., regardless ofthe specific currentvalues, at. current densities from about 2 to about-15r amperesvper square foot. Y

2. In a method of controlling the electrodefA position of copper from an acid copper sulfate plating` bath ontov aworkpiece of the type -in which the copper is normally deposited in greater concentrations in some areas of the-work than in others by direct current plating;V the stepwhich comprises electrodepositing copper onsaid workpiece from an acid bath-by a superimposed alternating-direct plating current comprising an alternating current to direct current ratio from about 2 to about 4 volts R. M. Si alternatingcurrent having, a; frequency within the-limits from about 25 cycles to about 60 cyclesto about 2.5zvolts.-

D. C., regardless of the specific-,current values, at` current densities from, about 2 to about 15 amperes pery square foot for a period-of time-withinthe range from. 8 seconds'to abputll-il minutes- EL CHESTER..

References Cited in the file of this patent- UNITED-STATES. PATENTS,

Number NameV Date 1,918,605 Jones July 18,1?933 2,063,760 SchuieinA Dec. 8; 1.936'- 2,443,599 Chester June 22, 1948-V 2,515,1A92 Chester July 18, 1950 Transactions of American Electrocheznica/l` Society, vol..41 (1922), pp. l51l80. 

1. IN A METHOD OF CONTROLLING THE ELECTRODEPOSITION OF COPPER FROM AN ACID COPPER SULFATE PLATING BATH ONTO AN ELECTRICALLY CONDUCTING OBJECT OF A TYPE IN WHICH THE COPPER WOULD NORMALLY BE DEPOSITED IN GREATER CONCENTRATIONS IN SOME AREAS THAN IN OTHERS BY A DIRECT PLATING CURRENT; THE STEP WHICH COMPRISES ELECTRODEPOSITING THE COPPER BY SUBJECTING SAID OBJECT IN AN ACID PLATING BATH TO A PLATING CURRENT HAVING AN ASYMMETRIC ALTERNATING WAVE FROM COMPRISING AN ALTERNATING CURRENT SUPERIMPOSED ON A DIRECT CURRENT IN THE 